America's air traffic control systems are vulnerable to cyber attacks, and support systems have been breached in recent months to allow hackers access to personnel records and network servers, a new report says.

The audit done by the Department of Transportation's inspector general concluded that although most of the attacks disrupted only support systems, they could spread to the operational systems that control communications, surveillance and flight information used to separate aircraft.

The report noted several recent cyber attacks, including a February incident, in which hackers gained access to personal information on about 48,000 current and former FAA employees, and an attack in 2008 when hackers took control of some FAA network servers.

Auditors said the Federal Aviation Administration is not able to detect potential cyber security attacks adequately, and it must secure its systems better against hackers and other intruders.

"In our opinion, unless effective action is taken quickly, it is likely to be a matter of when, not if, ATC (air traffic control) systems encounter attacks that do serious harm to ATC operations," the auditors said.

In response to the findings, FAA officials stressed that the support systems and traffic control networks are separated. They agreed, however, that more aggressive action should be taken to secure the networks and secure high-risk vulnerabilities.

According to the report, the FAA received 800 cyber incident alerts during the budget year that ended September 30, 2008, and more than 150 were not resolved before the calendar year was over.

Fifty of those, the auditors said, had been open for more than 3 months, "including critical incidents in which hackers may have taken over control" of some computers.

Officials tested internet-based systems that are used to provide information to the public such as communications frequencies for pilots, as well as internal FAA computer systems.

The tests found almost 4,000 "vulnerabilities," including 763 viewed as "high risk." The vulnerabilities including weak passwords, unprotected file folders and other software problems.

The weaknesses could allow hackers or internal FAA workers to gain access to air traffic systems, and possibly compromise computers there or infect them with malicious codes or viruses, the audit warned.

Such software gaps, the report said, are "especially worrisome at a time when the nation is facing increased threats from sophisticated nation state-sponsored cyber attacks."

In its response to the audit, the FAA said corrective actions already are being taken, and others should be in place in the coming months.

The audit is the latest in a series of reports and warnings about weaknesses in the US government's computer networks, including revelations that spies have hacked into the US electric grid and a military aircraft program was breached, although classified information was not compromised.

The Obama administration, meanwhile, is wrangling over a recently completed review of the nation's cybersecurity, which is expected to detail how the United States should manage and secure its networks.